1. Field of the Invention
This invention relates to a tuner and a radio receiver using the tuner which enables accurate tracking adjustment and can prevent any radiation of unnecessary frequencies from a local oscillator of a front end circuit.
2. Description of the Related Art
In a radio receiver having an FM receiver function, the unit composed of an FM receiving high frequency amplifying circuit, a mixing circuit and a local oscillator is called a front end. In recent years, the front end circuit is mounted on a single substrate and is shielded by a shield plate, particularly from radio output signals, to prevent interference or oscillation.
In order to introduce antenna input, the front end circuit has structures of FIGS. 4 and 5 of the accompanying drawings as in the case of a radio receiver for a vehicle.
In the structure of FIG. 4, an antenna jack 70, to which a coaxial cable 72 is connected, is attached to a rear surface 76 of a chassis 74. The coaxial cable 72 is fastened by a screw to the rear surface 76 of the chassis 74 using an attachment member 80. At a region corresponding to a cutout 82 in the attachment member 80, an insulating surface layer of the coaxial cable 72 is peeled to expose internal reticular wires 72a. The exposed reticular wires 72a are soldered to the attachment member 80 to connect thereby the antenna jack 70 to the ground. The signal terminal of the coaxial cable 72 is soldered to a pin on the substrate in the chassis 74.
In the structure of FIG. 5, an antenna jack 90 is attached directly to a main circuit substrate 100. The antenna jack 90 is composed of an earth tube 90a and a signal terminal 90b insulated from the earth tube 90a. The signal terminal 90b of the antenna jack 90 is soldered to a signal pattern of the main circuit substrate 100 while the earth tube 90a is supported by an attachment member 92 fixed on the main substrate 100. The attachment member 92 includes force-in plate 94 supporting the earth tube 90a forced into the plate 94. The attachment member 92 also has an attachment strip 96 which is fastened to the rear surface 76 of the chassis 74 by a screw 98. On the main circuit substrate 100, a front end pack 110 is mounted. The front end pack 110 includes at least an FM high frequency amplifying circuit, a mixing circuit and a local oscillating circuit, which are all mounted on a substrate 112 supported in a shield case 114.
The conventional structures of FIGS. 4 and 5, could not effectively prevent the transmitting output from the local oscillating circuit in the front end pack from leaking unnecessary radiation.
According to the structure of FIG. 4, the earth terminal of the coaxial cable 72 is connected to the chassis 76 by the attachment member 80. The potential of the earth pattern of the front end circuit will be finally the ground potential different from the potential of the chassis 76. This is because a relatively long earth line is formed between the earth of the front end circuit and the chassis 76 and has a high impedance. Further, the signal terminal of the coaxial cable 72 also is connected to the antenna input terminal of the front end circuit by a relatively long signal line.
Partly since the impedance in the earth line of the antenna input is increased, and partly since the signal line for the antenna input also is long, the oscillation output of the local oscillating circuit would be radiated out from such long signal line so that so-called unnecessary high frequency radiation would occur, thus causing jamming.
According to the structure of FIG. 5, the signal line and earth line for antenna signals are formed along a long pattern formed in the substrate 100 as a thin film so that the impedance of the earth line is high. In practice, therefore, if reticular wires 116 were not soldered between the shield case 114 of the front end pack 110 and the earth tube 90a of the antenna jack 90, the above-mentioned unnecessary high frequency radiation could not be prevented.
Another problem with the conventional structures of FIGS. 4 and 5 is that tracking adjustment solely for the front end circuit itself cannot be conducted under the same conditions as actual using conditions. Specifically, in either structure of FIGS. 4 and 5, while adjustment is made solely for the front end circuit, the antenna jack 70 or 90 to be used in an actual apparatus assumes a disconnected posture. Consequently, it was inevitable that a pin for antenna input was mounted on the substrate of the front end circuit to conduct adjustment while antenna input was made to the pin.